Which of the following is the most likely reason a pesticide application fails to control a pest?

• A. The pest developed resistance to the pesticide
• B. The LD50 of the pesticide is too low
• C. The rate of product applied was too high
• D. The restricted-entry interval was not followed

Answer: (A)

Pest resistance to the pesticide is the most likely reason. When the same chemical is used repeatedly, some individuals in the pest population naturally survive and pass those tolerant traits to their offspring. Over time, the population shifts toward resistance, so even properly labeled applications fail to control the pest as effectively as before. This is why rotating modes of action and using integrated pest management strategies help delay resistance and maintain control.

Notes on the other ideas: a very low LD50 means the chemical is highly toxic to the pest, which would generally improve, not hinder, control. applying at too high a rate might cause crop injury or safety issues but wouldn’t explain a loss of effectiveness due to the pest’s changing biology. not following a restricted-entry interval touches safety and compliance, not the pest’s resistance to the product.